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Inrush current in high-temperature superconducting (HTS) transformers is a little-studied phenomenon. After connecting to the power grid, a current flow through the windings which exceeding the critical current value of the superconducting tape. It may cause significant overheating and thermal damage of winding. The purpose of the study is to develop a mathematical model for calculating inrush current pulses in a HTS transformer and its verification by physical experiments. To achieve the goal of the study, a mathematical model has been developed that accurately represents the electromagnetic and thermal transient processes after HTS transformer is turned on at idling or under load. The model considers the critical parameters of the HTS tapes, the process of heating and cooling of the windings, quench characteristics, and the electrical and magnetic parameters of the transformer. Good compliance of the experimental results and mathematical modeling with a deviation of 1.99 % allowed us to verify the model. The most important result is the creation of a mathematical model of the HTS transformer at the moment of connecting to power grid. This model represents the temperature changing of the windings during the loss of superconductivity. The developed model can be used in the analysis and modeling of inrush current in designed and operating HTS transformers for any power. The obtained results are significant for determine the optimal starting characteristics, geometric and electrical parameters of HTS transformers. The proposed methods for reducing the inrush current ensure safe and reliable operation of the HTS transformer when switched on at idling or under load.

The study's aim is to substantiate and employ materials with high technical characteristics for the active elements of a transformer, particularly in the context of increased operating frequencies. To accomplish this objective, the following tasks were addressed: An analysis and synthesis of the properties of a magnetic core composed of amorphous iron and superconducting windings in a high-temperature superconducting transformer (HST) were conducted. Dependencies and graphs illustrating the impact of frequency increase on heat losses related to hysteresis and eddy currents were derived. These losses occur within the magnetic circuit. The study also demonstrates the relationship between reducing size and weight parameters and material consumption of windings in high-temperature superconducting (HST) tapes. Current densities in these tapes can reach up to 500 A/mm². The most noteworthy outcome, taking advantage of superconductivity in the HST, is that HST acts as an ideal diamagnetic material. Additionally, the windings exhibit high electrical conductivity. This resolves issues related to current displacement to the conductor's surface, commonly known as the "skin effect," and effectively addresses concerns about increased resistance in transformers and electrical machines. The absence of the skin effect in HST conductors has been rigorously demonstrated using Bessel functions. The significance of these results is in enhancing the efficiency of transformers operating at high frequencies. This is achieved by synthesizing the properties of the transformer's active elements, which include an amorphous iron magnetic core, HST windings, and a dielectric medium utilizing liquid nitrogen.

Today, high-temperature superconducting (HTS) current limiters and transformers allow to limit the surge short circuit current during failure without negatively affecting on the power grid com-plex at the normal operation mode. However, the transition of a superconductor to a resistive state at the moment of current limitation can cause significant heat generation, which can destroy the transformer windings. The research goal is to provide optimal technical characteristics of the HTS transformer to achieve effective short circuit current limitation and prevent thermal breakdown of its windings. To achieve this goal, a mathematical model of a HTS transformer was developed. The presented method considers the material type and geometry of the superconducting tape, the critical parameters of the superconductor (current and temperature), the parameters of the cryogenic liquid, dependence of the resistance and heat capacity of the HTS tape layers on temperature. The simulation model was created in the Matlab/Simulink software. The most important result is the possibility of obtaining optimal elec-trical and thermal parameters of the HTS transformer windings during the short circuit current limitation, as well as ensuring the thermal stability of the superconducting tape at the quench moment. The obtained results are significant in the design and operation of HTS transformers. For efficient and safe operation in the current-limiting mode, it is necessary to take into account heat generation on the transformer windings. It is important for the superconductor returning to the superconducting state without causing significant overheating of the windings.

High temperature superconducting (HTS) transformers have many advantages over conventional transformers in terms of volume, weight, and total efficiency. However, fault conditions limit the electro-thermal performance of an HTS transformer. Based on this, we investigate the thermal modeling of the HTS transformer in the fault-current state. For thermal modeling, heat transfer and parameters affecting it such as surface roughness and bubble behavior have been considered. At first, an accurate thermal model for a 10 kVA HTS transformer has been prepared. Experimental results show that the penalty of this model is about 2.5 K in the hot-spot point (HSP) temperature in fault-current conditions. In a second step, the impact of a roughness on the heat transfer and mass transfer rate in nucleated boiling mode is investigated. Results show that surface roughness decreases the HSP temperature by more than 10 K in fault-current conditions. These results have significant impacts on protecting HTS windings against quenching. [ABSTRACT FROM AUTHOR]

In order to improve the ability of superconducting transformer on limiting fault current, the parallel stainless steel & REBCO tapes are used as the winding materials. Firstly, the critical current of an individual REBCO tape and the parallel stainless steel & REBCO tapes under fault current are measured experimentally to compare their ability of limiting fault current. Then the current distribution in the parallel stainless steel & REBCO tapes under different fault currents are simulated. Lastly, a 6 kV/400 V (120kVA) HTS transformer is designed, and the fault current characteristics of wingdings are simulated based on finite element method (FEM). Experimental and simulation results show that the parallel stainless steel tape can improve the current limiting performance of the windings, which is of important referential value for the design of HTS transformer. [ABSTRACT FROM AUTHOR]

The paper provides the results of the experimental and computational study of the processes occurring in high temperature superconducting transformer windings while secondary winding is short-circuited. The obtained mathematical simulation matches closely with the experimental results. The temperature variation curves for superconducting windings were analysed, and conclusions were made on the necessity of changes in HTS transformer design, namely the necessity of windings heat-insulation from each other and adding a high-resistance coating material for HTS wire in HTS transformer primary winding. [ABSTRACT FROM AUTHOR]

In Offshore Substations (OSs), using High Temperature Superconducting (HTS) transformers instead of conventional power transformers to reduce operational and construction limitations such as weight, capacity, marine pollution, and future planning can be considered as desirable. HTS transformers can reduce operating system costs in wind power plant development programs. However, OSs have specific operating conditions in fault mode that should be considered in the design HTS transformers. In this paper, a 250 MVA 33/220 KV HTS transformer is designed, and electromagnetic analysis is done in normal conditions to calculate AC loss and thence to design the cooling system. The second step, thermo-flow analysis is done by Finite Element Method (FEM) to calculate the temperature distribution of the windings. Increasing turbulence of inlet flow and creating a porous coating on the tapes are used in this transformer to increase the heat-transfer rate. lastly, the specifications of the HTS transformer are compared with the conventional transformer to give useful information to OS designers to investigate using novel construction ideas such as floating OS by HTS transformer. [ABSTRACT FROM AUTHOR]

HTS transformers can provide lower loss and higher efficiency with smaller size and lighter weight compared with traditional transformers. In this paper, we designed a 500 kVA HTS transformer and developed its simulation using finite element software based on the H -formulation, and the simulation incorporates a heat model and E-J power law. The simulation shows that there is a large radial flux leakage at the end of the low-voltage (LV) windings (superconducting windings), leading to a reduction in critical current density and an associated increase in joule loss in the LV winding. In order to reduce joule loss, various LV winding structures were designed. The results demonstrate that either increasing the axial distance between the coils at the LV winding end or installing a flux diverter outside the LV windings can effectively diminish the radial flux leakage at the LV winding end, consequently reducing joule loss in the LV winding. The impact of the size, position and relative permeability of the flux diverter on radial flux leakage and joule loss are also studied. These structure optimizations and the corresponding effects have important significance to the design of HTS transformers. [ABSTRACT FROM AUTHOR]

Manufacturers and consumers of transformers rather conservatively concern all innovations in transformer manufacture since in power distributive transformers are inherent long term of the operation, essential cost and a purchasing cycle. Progress of perfection of a design of the transformer is substantially defined now by creation new and perfection used conduction, magnetic and insulating materials.Regarding creation new conduction the materials applied by manufacture of mains transformers, it is possible to allocate high-temperature superconducting materials and, accordingly, the HTS transformers created with their application. Pre-production models of HTS transformers with use HTS of a tape of 2nd generation on the basis of superconducting ceramics GdBCO are already made and are tested. Development of manufacturing techniques of the hightension cable has allowed to create a cable with isolation from cross linked polyethylene (XLPE) and to apply it in the field of transformer technologies by working out of the new dryformer.Regarding creation of new magnetic materials the special role is taken away so-called, to the nano-crystal alloy which features have laid down in a basis of creation of new type of amorphous core transformers. Now in the world works on working out of tapes from the amorphous alloys possessing improved magnetic properties and cheapness, in particular Fe(Co)SiBPCCu and 2605SA1 are conducted. As tendencies in change of type of the core and its design are observed. More and more wide application at manufacturing of power three-phase transformers small and a mean power is found by the strip-wound cores which design is executed on technology UNICORE. As interest to a design of cores of the electrotechnical steels executed from different types and received the name “Sandwich” is shown.Regarding perfection of used insulations materials, it is possible to note application of electrotechnical gas (SF6) in transformers in circuit-breaker oil replacements. Such transformers have received the name gas-insulated transformers, and their obvious advantage are ecological compatibility and fire safety.Now it is much spoken about creation of so-called Smart Grid, and their transition in the future to the most effective systems of a highest level called now Microgrid. Certain demands are made to the specified electric systems: maintenance optimum capacity factor and high electrical power factor. The resulted requirements, at present, can be realised only new semiconductor transformers, such as the “Sen” transformer.

One of critical issues for HTS transformers is achieving sufficiently low AC loss in the windings. Therefore, accurate prediction of AC loss is critical for the HTS transformer applications. In this work, we present AC loss simulation results employing the H -formulation for a 1 MVA 3-Phase HTS transformer. The high voltage (HV) windings are composed of 24 double pancakes per phase wound with 4 mm – wide YBCO wire. Each double pancake coil has 38 ¼ turns. The low voltage (LV) windings are 20 turn single-layer solenoid windings wound with 15/5 (15 strands of 5 mm width) Roebel cable per phase. The numerical method was first verified by comparing the numerical and experimental AC loss results for two coil assemblies composed of two and six double pancake coils (DPCs). The numerical AC loss calculated for the transformer was compared with the measured AC loss as well as the numerical result obtained using the minimum magnetic energy variation (MMEV) method. The numerical AC loss result in this work and experimental result as well as the numerical result using MMEV at the rated current agree to within 20%. Further simulations were carried out to explore the dependence of the AC loss on the gap between the turns of the LV winding. The minimum AC loss at rated current in the 1 MVA HTS transformer appears when the gap between turns is approximately 2.1 mm turn gap in the LV winding. This is due to the change of relative heights between the HV and LV windings which results in optimal radial magnetic field cancellation. The same numerical method can be applied to calculate AC loss in larger rating HTS transformers. [ABSTRACT FROM AUTHOR]

With the mature of manufacture process and technology of high temperature superconductors, the critical current and stability are gradually increased. High temperature superconductors could be used in transformer, current limiter, generator, magnet and etc. This paper focuses on development and characteristic tests of a single-phase high temperature superconducting (HTS) power transformer with capacity of 330 kVA by using GdBCO tapes, which is 1/3 model of a 1 MVA / 10 kV / 0.4 kV HTS transformer. The specifications of iron core, HTS windings and cryostat are described in detail. The iron core is made of silicon steel plate. The arrangement of HTS windings are based on experimental and simulated results. The cryostat with a room temperature bore is manufactured using nonmetallic materials. Several characteristic tests and insulation tests are performed in liquid nitrogen of 77 K. The efficiency and AC loss at rated load is 99.90% and 243.7 W, respectively. In addition, an overload test was also performed. [ABSTRACT FROM AUTHOR]

This paper focuses on conceptual design and electromagnetic analysis of a single-phase high-temperature superconducting (HTS) power transformer with a capacity of 300 kVA by using GdBCO tapes, which is one-third model of a 1 MVA/10 kV/0.4 kV HTS transformer. The conceptual design consists four parts: iron core, HTS windings, current lead, and cryostat. In the electromagnetic optimization of the 330 kVA HTS transformer windings via finite element method, the influence of winding configurations on the radial magnetic field and the current nonuniformity are considered. The details about the design and optimization of radial magnetic field will be discussed in this study. [ABSTRACT FROM PUBLISHER]

This paper focuses on the comparison of the characteristics among three different types of high temperature superconducting (HTS) transformers. The iron core structures of the three types of HTS transformers are air core, partial core, and full core. The iron cores are made of oriented silicon steel sheets. The primary windings are four layers connected in series, and the secondary windings are nine double pancake coils connected in parallel. Fundamental tests, such as critical test, no-load test and load test, are performed in the environment of liquid nitrogen. The current nonuniformity of HTS partial core and full core transformers in secondary windings are also confirmed by the load test. Finally, we also investigate the characteristic of fault current limiting by sudden short-circuit tests. The experiment results show that the structure of the iron core have significant effects on the characteristics of HTS transformers. The results of this paper could support with useful data for the design and selection of HTS transformers. [ABSTRACT FROM PUBLISHER]

In recent years, using high-temperature superconductor (HTS) transformers in transportation and electrical power systems has attracted much interest. The major advantages of HTS transformers compared with conventional ones are much smaller size and volume and lighter weight in confined spaces of urban areas or transportation devices. One of the most critical parameters to design, fabricate, and commercialize HTS transformers is hysteresis loss in HTS tapes. Using numerical methods, the accurate value of hysteresis loss and efficiency can be calculated or estimated in HTS transformers. In this paper, two innovative techniques to reduce the leakage flux and thereby hysteresis loss in HTS transformers have been proposed. In the first technique, the concentric windings of HTS transformer are apportioned into a certain number of sub-windings. In the second one, with optimizing the distributive ratios of sub-windings, the windings become unsymmetrical. In order to demonstrate the effectiveness of proposed techniques, an accurate and advanced numerical method based on finite element analysis has been used to design four symmetrical and unsymmetrical primary (P)-secondary (S)-P-S-P and P-S-P-S-P-S-P winding schemes. The simulation results prove the effectiveness of both techniques in reducing the radial and axial leakage flux magnitudes and thereby hysteresis loss in HTS windings. [ABSTRACT FROM AUTHOR]

• Hysteresis losses calculation in a 3-phase HTS transformer has been developed for harmonic load. • A comprehensive accurate fast analytical algorithm has been proposed for the first time. • Finite Element Method has verified the results obtained by the proposed algorithm. • Hysteresis losses in a 3-phase HTS distribution transformer have been analyzed. • AC losses have been increased in the presence of the multiple-harmonic loads. Nowadays, the application of non-linear loads such as, rectifiers, electronic phase control, PWM drives, gives rise to a vastly increased level of harmonics in the power network. As such, the transport currents of transformers are non-sinusoidal alternating currents. On the other hand, AC losses are one of the most critical characteristics for the design and operation of high-temperature superconducting (HTS) transformers. The amount of hysteresis losses is a key parameter for the design of HTS windings affecting equipment, cooling cost, efficiency, and transformer economic feasibility. The computation of AC losses by traditional techniques, is suitable for the sinusoidal current condition. While, non-linear loads brings about different number of peaks and amplitudes in the current waveform passing through the HTS transformer windings. So that, in addition to the major hysteresis loop, a number of minor hysteresis loops are also formed changing the hysterical nature of the AC losses. Hence, this paper proposes an accurate, yet efficient analytical algorithm, for calculating hysteresis losses in a three-phase HTS transformer's windings in the presence of the harmonic loads accounting for the AC losses estimation. This is accomplished by performing a numerical Finite Element Method to verify the analytical results. The analysis of the hysteresis losses in an exemplary 315 kVA three-phase HTS distribution transformer for different harmonic loads, is another object of this paper. [ABSTRACT FROM AUTHOR]